Thin film forming equipment

ABSTRACT

A gate valve for a thin film forming apparatus. The gate valve includes two adjoining low-pressure chambers and a wall separating the two chambers. The wall includes an aperture and a thin plate for covering the aperture. The thin plate is movable in a direction substantially parallel to the plate surface. The gate valve further includes a voltage supply for applying a direct current between the thin plate and the wall.

This is a continuation of application Ser. No. 08/097,861, filed Jul.26, 1993, which is a continuation of application Ser. No. 07/990,549,filed Dec. 14, 1992, which is a continuation of application Ser. No.07/536,547, filed as PCT/JP89/00023 Jan. 11, 1989 all now abandoned.

FIELD OF THE INVENTION

The present invention relates to thin film forming equipment suitablefor the manufacture of ultra-high density integrated circuits.

BACKGROUND OF THE INVENTION

The progress of LSI technology is really amazing, and the degree ofintegration is increasing year by year. Taking an example in dynamicrandom access memory (DRAM), memory capacity has quadrupled in threeyears. The product development for 4-megabit DRAM has now beencompleted, and technical development is now directed to 16-megabit and64-megabit DRAMs. With the increase in the degree of integration, thedimension of the unit element is minimized, and the minimum dimension isdecreasing more and more from 1 μm to the order of submicron. Thestructure of various IC devices is basically composed of a laminatedstructure of various types of thin film. For example, the main portionof a MOS transistor is composed of 3-layer structure comprisingelectrode material, insulating thin film and semiconductor substrate.The capacitor, used for a memory cell of DRAM, has semiconductor 3-layerstructure with a high dielectric thin film sandwiched by upper and lowerelectrode materials. A non-volatile memory element has a 5-layerstructure composed of semiconductor substrate, insulating thin film,electrode material, insulating thin film, and electrode material. Thus,the thin film laminated structure is a structure dominating the mostimportant characteristics of the device. Because the thickness of thesethin films is increasingly becoming thinner with the miniaturization ofthe device, the characteristics of these thin films are the importantfactor to determine the characteristics, yield and reliability of LSI.Therefore, the keypoint for the actualization of ultra-high integrationis the technique to form high quality thin film and to produce thelaminated structure of thin film with high reliability. Further, thisprocess requires low temperature instead of high temperature of900°-1000° C. as used at present. For example, to produce a capacitorstructure using aluminum as a lower electrode, the temperature to forman insulating film and electrode on it must be lower than the meltingpoint of aluminum (about 630° C.) for instance, 500°-550° C. or less--ormore preferably, 400° C. or less. For accurate control of N-type orP-type impurities, it is necessary to reduce the process temperature to700° C. or less.

In the following, description will be given on the method to produce aconventional type thin film laminated structure, taking an example inthe manufacturing process of a DRAM memory cell. FIG. 23 is a schematicdrawing showing the sectional structure of a memory capacitor unit of aDRAM memory cell as formed by the conventional technique. To producethis structure, field oxide film 2303 is formed on silicon substrate2302, and the surface of silicon substrate 2302 of memory capacitorforming portion 2301 is exposed. Then, SiO₂ film 2304 of about 100Å isformed by thermal oxidation at 900° C. Thereafter, polycrystal siliconthin film 2305 is deposited by CVD method, and a memory capacitor isproduced through the patterning into the predetermined shape. In thisprocess, after the surface of silicon substrate 2302 is exposed byetching with dilute HF solution, the wafer is placed into a thermaloxidation furnace to grow the oxide film. After the wafer is taken outof the furnace, it is placed into a CVD apparatus and polycrystalsilicon film 2305 is deposited, and this is processed to thepredetermined pattern. Namely, the interface of each thin film comesinto contact with atmospheric air because each thin film composing thelaminated structure is formed in a separate apparatus in the normalprocess. For this reason, the interface is contaminated by adsorption ofadsorptive contaminants in the gas in the atmospheric air, and thisresults in the instability and variation of isolation voltage or othercharacteristics of thin film oxide film. The oxide film of 100Å is theinsulating film to be used for 1-megabit DRAM. For 4-megabit or16-megabit DRAM, it is necessary to have a thin film of 50Å or thinner,and the problem of interface contamination is an important and seriousissue for the decrease of isolation voltage or the reliability of thinfilm oxide film. In some cases, silicon nitride film (Si₃ N₄) thin filmhaving a higher dielectric constant than thermal oxide film of Si (SiO₂)is used as capacitor insulating film 2304 of DRAM. Because it is verydifficult to form Si₃ N₄ film through direct nitriding of silicon, Si₃N₄ film deposited by LPCVD method is used. Normally, thin film formed bydepositing has poor characteristics in the interface with silicon andthere are more defects such as pinholes. Accordingly, after the siliconsurface is processed by thermal oxidation, the characteristics ofinterface with Si is improved by depositing Si₃ N₄ film, and pinholesare filled up by thermal oxidation after Si₃ N₄ film is deposited. Insuch a process, the final capacitor structure is a 5-layer laminatedstructure composed of Si, SiO₂, Si₃ N₄, SiO₂ and poly-Si (polycrystalsilicon), this means that there are four interfaces to be exposed toatmospheric air, and it is very difficult to prevent the contamination.Also, it is almost impossible to carry out the process at lowtemperatures because thermal oxidation is usually performed at thetemperature of 850°-900° C.

To produce and utilize ultra-high integration in the future, it is veryimportant to establish the technique to produce the laminated structureof very thin film at low temperatures and with no possibility to inducecontamination on the interface.

SUMMARY OF THE INVENTION

To attain such purposes, the present invention offers a gate valve forthin film forming equipment suitable for the manufacture of ultra-highdensity integrated circuits.

The present invention proposes a gate valve for thin film formingequipment comprising two adjoining low-pressure chambers, a wallseparating the chambers and having a freely openable and closeableaperture, a thin plate for covering the aperture and having a platesurface, a device for moving the thin plate in a direction substantiallyparallel to the plate surface, and a voltage supply for applying adirect current voltage between the thin plate and wall. The platesurface may be polished and disposed facing a peripheral part of theaperture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of the equipment of an Embodiment of thepresent invention;

FIG. 2 is a sectional view of the equipment to show another embodimentof this invention;

FIGS. 3 to 8 are schematic drawings of the sputter chamber;

FIGS. 9 to 12 represent schematic drawings to show an example ofhigh-speed gate valve;

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, the embodiments of this invention will be described inconnection with the drawings.

FIG. 1 is a system diagram of the thin film forming equipment showing anembodiment of this invention. The equipment comprises four reducedpressure chambers (process chambers), each provided with differentfunctions, as a sputter chamber for metal thin film 101a, a sputterchamber for insulating thin film 101b, a cleaning chamber 101c, and anoxidation chamber 101d. 102 is a wafer load chamber and is used to sendthe wafer to the equipment, 103 is an unload chamber and is used to takethe wafer out of the equipment. 104 is called a transport chamber and isused to transport the wafer to each predetermined chamber of the above 4process chambers. For the transport of wafer 106c, for example, waferchuck 114 of electrostatic adsorption type is used. Specifically, thewafer is set on wafer holders 107a-d of the predetermined chamber usinga magnetic floating type transport mechanism. These wafer holders 107a-dhave such structure that, wafers are adsorbed and maintained byelectrostatic chuck, and, after the gate valves 105a-105d of thecorresponding chamber (gate valve 106a of process chamber 101a notshown) are opened, the entire wafer holder moves upward to place waferinto the predetermined process chamber and the gap between processchamber and transport chamber is sealed in an airtight manner. In FIG.1, silicon wafer 106a and wafer holder 107a are set on the sputterchamber 101a for metal thin film.

108a-108d are called target chambers, and target materials 109a-109d canbe exchanged without breaking the vacuum condition. In each target, RFpower supplies 111a-111d are connected through the tuning circuits110a-110d, and RF power supplies 113a-113d are connected to waferholders 107a-107d through each of the tuning circuits. Although notshown in figure, a vacuum exhaust unit is connected to each of thesechambers.

In the structure shown in FIG. 1, target and wafer are placedface-to-face in upward and downward directions with wafer down andtarget up, but the arrangement is not limited to this structure. Forexample, heavy target may be placed at a lower position, and the wafermay be at upper position. In so doing, it is possible to prevent thedropping of the target in case adsorption power is temporarily weakeneddue to transient fluctuation of supply voltage to the electrostaticchuck when the target is maintained by electrostatic chuck. However,when small pieces of wafer or dust are attached on the wafer, it fallson the target, and target may be contaminated, or the quality of thinfilm formed by sputtering may be extremely deteriorated. Therefore, itis more effective to have the structure where target 109a and wafer 106aare placed face-to-face in a left-right direction as shown in FIG. 1.This eliminates the problem of dust attachment on the wafer and target.

Such is an outline of the arrangement of this equipment and theformation of multilayer thin film structure. In the following,description will be given on the details of each component of theequipment, and the details of the formation of multilayer thin filmstructure to explain the operation and the effect of the presentinvention.

FIGS. 3 and 4 are the detailed representations of the structure ofsputter chamber for metal thin film 101a, which is one of the processchambers. In the figures, transport chamber 104, target chamber 108a,wafer holder 107a, gate valve 105a, target 109a, target holder 401, etc.are also shown.

To obtain high quality thin film by sputtering, it is important tocompletely exclude the intermingling of impurities such as moisture intothe film forming process. For this purpose, the Ar gas method must beadopted as described above. In addition, it is important to reduce thedegasification from chamber material or from the gas pipe materialsurface. The wall material 402 of the chamber of the equipment of FIG. 3is made of SUS304L or SUS316L. It is important to process the surfacewith the treatment to reduce the adsorption of H₂ O molecules and tofacilitate the desorption. As to such treatment, there is the followingmethod:

First, mirror polishing is performed not accompanied with processedlayer of the stainless steel surface. To the inner surface of the pipes,for example, electropolishing is applied to the inner surface of thechamber.

Next, purging is performed with Ar or He with a moisture content of 1ppb or lower. Further, temperature is increased to about 400° C. andpurging is performed. After H₂ O molecules adsorbed on the surface areremoved almost completely, pure oxygen with a moisture content of about1 ppb or less is supplied, and temperature is increased to 400°-500° C.and the inner surface is oxidized. Oxide film obtained through thermaloxidation of the stainless steel surface has a higher corrosionresistant property to corrosive gases such as HCl, CI₂, BCI₃, BF₃, etc.compared with passive film formed by the conventional method usingnitric acid, etc. Further, it has many excellent features such as lowsurface occlusion of water molecules harmful to the process, highdesorption property, etc.

Next, description will be given on oxide film obtained through oxidationof a stainless steel surface. Table 2 summarizes film thickness andrefractive index of an oxide film formed on the surface when SUS316L andSUS304L are oxidized by ultra-high purity oxygen. It is expressed as therelation between oxidation temperature and time. It should be noted thatoxide film thickness does not depend on time, and it is determined onlyby temperature. This suggests that the oxidation of SUS is proceeding bythe processes as described by the models of Cabrera and Mott. In otherwords, when temperature is controlled to a constant level, oxide filmgrows to the desired thickness. Thus, it is possible to produce theoxide film having uniform thickness and high density without pinholes.

                  TABLE 1                                                         ______________________________________                                        Moisture Content Desorbed From Sample Pipe                                    (Temperature-dependent property)                                              ______________________________________                                        Temp. (°C.)                                                                       Room temp. - 120                                                                            120-200  200-300                                     Sample     420           600      860                                         Electropolished                                                               pipe                                                                          Pipe passivated                                                                          750           630      990                                         by nitric acid                                                                Pipe by this                                                                              25            70      100                                         invention                                                                                                       Unit: ppb                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Film Thickness of Passivation Film                                                          SUS316L    SUS304L                                                                  Film           Film                                       Oxidation                                                                              Oxidation  thick- Refrac- thick-                                                                             Refrac-                               temp.    time       ness   tive    ness tive                                  (°C.)                                                                           (hr)       Å  index   Å                                                                              index                                 ______________________________________                                        400      1          114.0  2.71    78.8 3.26                                           4          110.9  2.87    74.2 3.41                                  500      1          125.7  2.93    95.8 3.60                                           2          126.1  2.91    95.2 3.50                                           4          126.8  2.96    91.3 3.81                                  550      1          130.9  3.02    102.9                                                                              3.56                                           4          141.8  3.13    110.9                                                                              3.76                                  ______________________________________                                    

This means that Fe oxides are present near the surface, while there aremore Cr oxides near the interface between the oxide film and SUSsubstrate, showing a 2-layer structure. This is also supported by theresults of energy analysis of ESCA spectrum, i.e. chemical shift due tothe formation of oxides is observed in Fe near the surface, while thisdisappears in deeper portions where chemical shift due to the formationof Cr oxides is seen. There has been no report so far describing that a2-layer film was formed by the oxidation of SUS, and very little isknown about the mechanism of the excellent corrosion resistent propertyof the reduced pressure equipment of this invention and the desorptionproperty of adsorbed gas, whereas it is attributable to the formation ofa dense 2-layer film. Here, the film with thickness of about 100Å wasused, while the same effect is obtained with the film with thickness of50Å or more. However, it is preferable that the film thickness is 50Å ormore because pinholes are generated when film thickness is less than 50Åand the corrosion resistant property is decreased.

To form a dense oxide film, it is important to remove the layerdegenerated during processing of SUS surface and to flatten the surface.In the present embodiment, the material with surface phase of Rmax at0.1 to 0.7 μm was used, whereas it is known that good passivation filmcan be obtained when the maximum difference between peak and bottom ofthe irregularities are within a circle with radius of 5 μm is up to 1μm.

By the passivation process as described above, it was possible to makethe chamber match the ultra-high vacuum condition and also to providefull corrosion resistant property against corrosive gas. This has madeit possible to remove the deposited reaction products attached on wallsurface by passing chlorine type gas for the cleaning inside the chamberand by increasing chamber temperature. Because the inner surface of thechamber is flat and is provided with a dense passivation film, theadhesive power of the attached objects is very weak, and thisfacilitates gas etching. If such cleaning is not required, it is alsoeffective to adopt an aluminum alloy chamber, which is lightweight andsuitable for ultra-high vacuum condition.

For the target material, impurities are completely removed and gascomponents such as oxygen are removed by vacuum dissolving.

Next, description will be given of wafer holder 107a in connection withFIG. 3. The entire holder is connected to the outer wall of the chamberby bellows 403 and is movable upward and downward. With silicon wafer404 adsorbed on the electrostatic chuck electrode 405, it is moved upand down, and the wafer is placed into or taken out of the processchamber 101a. For example, when the wafer is to be inserted into theprocess chamber, the entire wafer holder 107a is moved up. By closelyfitting O-ring 406 on the flange surface 407 of the chamber, airtightsealing is provided between the process chamber 101a and the transportchamber 104. Here, the O-ring is adopted as a sealing material, while itis more effective to use a metal sealing with less degasification. Inthis case, it is preferable to use a metal seal, which has enoughelasticity to endure the repeated removing and inserting operation andhas a high sealing property. For example, it is effective to use anO-ring made of elastic resin, which is placed in a metal ring in theshape of a plate spring. Such a ring should be made of Al, Ni, SUS316L,Ni-coated stainless steel, etc., which shrinks within the range ofelasticity (i.e. not to cause plastic deformation). In this case, asealing surface is maintained by the contact of the metal surface. (Ifthis surface is made as a mirror surface with Rmax of 0.2 μm or less, itis possible to reduce the leakage.) Because the pressing force tomaintain the seal is supplied by a rubber O-ring, it is possible toprovide high airtightness and repeated operation.

It is preferable to furnish the opening of the ring on the side with thelower degree of vacuum. Further, it is preferable to provide a notch tocommunicate with the interior and the opening because this prevents thestagnation of gas toward the interior. This notch is crushed whenpressure is applied on the ring, and the interior is sealed. The platemay be furnished with a hole communicating with the interior.

When wafer holder 107a is waiting in the transport chamber, the openingis sealed by gate valve 105a, and airtight sealing is provided betweenthe process chamber and transport chamber.

An O-ring 408 may be used for this sealing, whereas it is more effectiveto use a metal seal. Other sealing means may be used if completeairtightness is assured.

In any case, it is important when vacuum sealing is provided by elasticmaterial to adopt such a structure that relative position of flangesurface 407 and flange surface 406 in FIG. 3 is determined regardless ofthe sealing component 406.

Specifically, it is preferable that the position is determined not bythe force to crush 406 but that relative position is always keptconstant by inserting a stopper 4201 not easily deformed as given in theenlarged view of FIG. 6. In so doing, the O-ring is always compressed bya constant force, and a stable sealing property is obtained. Of course,this also applies to the case where a metal ring is used instead of anO-ring. The stopper 4201 as described above may be formed directly whenflange surface (407 or 406') is fabricated, or a ring-like part may bemounted later. It is preferable to place this stopper at the side withlower degree of vacuum to prevent the formation of a dead zone on theside with a higher degree of vacuum.

It is also important to furnish a guide for upward and downward movementof 406', which moves up or down to prevent lateral deviation when thesealing component is compressed.

405 represents an electrostatic chuck electrode to maintain the wafer,and it is made of a metal such as stainless steel, Mo, Ti, etc. Aninsulating film 409 is formed on its surface. As this insulating film,Al₂ O₃ and AlN are formed on the surface of the electrode by plasmaspraying, and the surface is flattened by polishing. The thickness isabout 10-100 μm. By giving a potential difference of several hundredvolts between wafer 404 and electrode 405, the wafer can be held on thewafer holder by the force of 1 kg/cm² or more. Normally, when the waferis placed on the stage under vacuum condition, it comes into contactwith the stage only at 3 points, and it has been impossible to setaccurate wafer temperature. According to the equipment of thisinvention, the temperature of the wafer can be controlled with highaccuracy because the wafer is held on the stage with strong force.Potential is applied to the wafer by metal electrode 410. Electrode 410is naturally insulated from electrode 405 and is connected to the powersupply outside the system.

In the figure, a structure is adopted such that the potential of thewafer is applied at the center of the wafer by the electrode 410, whilea different structure may be adopted, where the potential is applied atthe peripheral portion of the wafer. Taking the potential at aperipheral portion is more advantageous for temperature control of thewafer because the uniformity in the plane can be attained more easilythan the case where a hole is furnished at the center of wafer holder.The entire electrode 405 is insulated from the chamber by insulator 411.Further, high frequency power with frequency f_(w) is supplied to theelectrode 405 from outside by 412.

FIG. 4 shows an example of connecting relation of electrode 405 andwafer 404 with external power supply. In FIG. 4, the same number refersto the same component as in FIG. 3. 4101 is a DC power supply for theelectrostatic chuck, and it gives a DC potential difference Vc betweenthe electrostatic chuck electrode 405 and the wafer 404 through highfrequency filter 4102, which cuts off high frequency and supplies onlythe DC potential. 4103 is an RF power supply having f_(w) of 100 MHz,and high frequency power is supplied to the wafer by the installationelectrode 412 through matching circuit 4101 and blocking capacitor 4105.By changing the power from this high frequency power supply 4103 withinthe range of several watts to several tens of watts, the DC potential ofwafer 404 can be set to the desired value. Or, by changing the matchingcondition of the matching circuit 4104, the DC potential of wafer can bechanged.

Even when the wafer surface is covered with insulating film such asSiO₂, the DC potential of the surface is almost the same as thepotential of the wafer. Because the capacitor capacity generated by SiO₂film is much higher than blocking capacitor 4105, self-bias due to highfrequency appears on both ends of this capacitor in almost all cases.

Therefore, by monitoring the potential of the wafer by a voltmeterthrough a high frequency filter, by feeding this back to RF powercontroller or to matching circuit controller, it is possible toaccurately control DC potential on wafer surface to a constant value. Bythe wafer potential thus determined, it is possible to accuratelycontrol the energy of ions coming to the wafer surface from the plasmato the desired value. On the other hand, high frequency with differentfrequency f_(T) (e.g. 13.56 MHz) is given to the target, and the energyis transferred to the wafer by the frequency f_(T) through capacitivecoupling of target electrode 415 and wafer holder electrode 405.

The circuit of 4106 is a circuit, which maintains high impedance to thefrequency f_(w) and short-circuits high frequency of frequency f_(T). Bythis circuit, DC potential of the wafer becomes controllable only byhigh frequency of f_(w) applied on the wafer holder. If L and C parallelresonance circuit is used with the relation:

    2 πf.sub.w =1/(LC).sup.1/2

it is open only to the high frequency of f_(w). To the other frequency,it is short-circuited if C is taken at high value, and the desiredfunction can be fulfilled. Because DC potential of a certain degree mustbe generated on wafer holder 405, a capacitor with sufficiently highcapacity is connected in series with the above LC parallel circuit.

In case electroconductive thin film is formed on the wafer surface andsuch a thin film is electrically connected with wafer, the potential ofthe wafer may be directly controlled by the DC power supply. In such acase, the wafer potential, i.e. the potential of wafer surface, can becontrolled by DC power supply 4108 when the switch 4107 is turned on.

In FIG. 3, 413 is a heater, and it is used to heat the electrode 405 ofthe wafer holder to the desired temperature. In this case, it ispossible to uniformly heat the wafer to the same temperature as theelectrode because wafer 404 is adsorbed strongly by the electrostaticchuck and to control wafer temperature accurately. 414 is a fiberthermometer, which measures temperature by sensing the luminance ofblack body radiation through the optical fiber. Thus, accuratetemperature measurement is assured without being disturbed by RF noiseand the like. Through the feedback of the results of measurement to theheater controller, accurate temperature control can be performed.

Here, description is given only to the heating system using a heater,while precise temperature distribution control can be performed throughdischarge heating by a number of plasma torches and through control ofthe discharged current.

One of the major features of the equipment by this invention is that nocontamination source such as wafer transport mechanism, wafer heatingmechanism, etc. is contained in the process chamber. This makes itpossible to maintain the process chamber in a very clean condition andto form high quality thin film. Further, because the heating mechanismis completely separated from the vacuum system and is placed inatmospheric air or under normal pressure, there is no need to worryabout contamination and uniform heating is assured.

Next, description will be given of the target holder 401 in connectionwith FIG. 3. 415 is a holder electrode and is made of metal such asstainless steel, Ti, or Mo. Its surface is covered with insulating thinfilm such as Al₂ O₃, AlNi, SiO₂, etc. and it works as an electrostaticchuck. To the metal target 109a, potential is given from backside by theelectrode 416, and it is electrostatically adsorbed and held bypotential difference between the electrode 416 and the holder electrode415. 420 is an insulating material to electrically insulate the holderelectrode 415 from the chamber. The other mechanism of the target holderis almost the same as that of the wafer holder 107a, and a detaileddescription is not given here. 417 is a magnet for magnetron discharge,and 418 is a pipe to pass cooling medium to cool the target. 419 is aground shield to prevent the sputtering of target holder. This groundshield may not be provided if the diameter of the target is larger thanthe diameter of the target holder. This ground shield was not describedin the explanation of the wafer holder 107a, but this may be providedsimilarly to the wafer holder. The target holder 401 has bellow seal 420as in the case of wafer holder 107a, and this facilitates upward anddownward movement. When the target is replaced, the entire wafer holdermoves up by shrinking bellows, and a gate valve (not shown) closes theopening as in the case of wafer holder.

The holder electrode 415 moves up again after adsorbing the target, andtarget stocker is rotated to have the notch 1104 under the holderelectrode. The holder 415 extends downward lower than the notch, insertsthe target into the process chamber 101a and is arranged as shown inFIG. 3.

It goes without saying that the load lock exchange mechanism of thistarget is used for other purposes than the sputter chamber for metalthin film of FIG. 3. In case the target is an insulating material,electroconductive material such as metal may be provided on backside. Ametal plate may be attached, or metal thin film may be formed bysputtering.

The target stocker may not be in the shape of a disk. Targets may bearranged linearly and may be slided to a lateral direction or to frontor back directions. They may be furnished separately for each of theprocess chambers (101a-101d) of FIG. 1, or a common stocker may beprovided, regarding all target chambers 108a-108d as one common chamber.The target in the target stocker may be maintained by gravity or it maybe mechanically supported by electrostatic chuck or by mechanical means.

Conventionally, the target has been forcibly cooled down from backsideof the target holder using a cooling pipe (FIG. 3; 418) becausetemperature is rapidly increased during sputtering. In order to havesatisfactory thermal transfer from target to holder, it was necessary tofix it on the holder, using screws or other means. Therefore, the targetis normally replaced by breaking the vacuum condition in the processchamber 101a, and it is usually replaced only when the target materialis worn out. Accordingly, none of the conventional sputtering filmforming equipment was provided with a mechanism to replace the targetwithout breaking the vacuum condition. In contrast, it is possible bythe equipment of this invention to easily replace the target because ofthe adoption of the target holder with electrostatic chuck. Thus, filmcan be formed from various types of materials in a single chamber, andthis extensively expanded the functions of the equipment. More importantis the fact that there is no need to fix the target by screws.

Because the target was fixed by screws usually from the surface of thetarget, screw material used to be a cause for contamination as it wassputtered. For this reason, screws were manufactured of the samematerial as target material, while this resulted in the difficulties infabrication. Particularly, when target material is purified up to thepurity of 6N-7N or more, each crystal size is increased to as high asseveral mm. During fabrication, it is split along crystal grainboundary, and it was impossible to perform complicated fabrication. Forthis reason, the material with a purity as low as about 3N has to beused for the parts such as screws which requires fine fabrication. Incontrast, in case of the equipment by this invention, there is no needto use screws because electrostatic adsorption holder is used. The shapeof target is also in simple disk type, and it is very easy to fabricate.Thus, it is possible to use the material of high purity, and theso-called cutting margin may be smaller in the fabrication of thetarget. This means effective utilization of expensive target material ofhigh purity, and this contributes greatly to the economy of fabrication.

In the past, thick material had to be used as target. However, when athick target is used, the target absorbs magnetic force in case a magnetis furnished behind it, and magnetic field of film making space isweakened. In contrast, a thinner target can be used in the equipment ofthis invention, and this makes it possible to produce thin film withgood film quality at high speed.

In FIG. 3, the potential difference between electrostatic chuckelectrode 415 and the target 109a is given by DC power supply 4109 asconnected through high frequency filter 4102. The potential of thetarget is directly supplied by 4110, and contact is kept at the centeras shown in the figure, while this may be taken from the peripheralportion of the target. For the power supply 4109, it is preferable touse battery backup in order to prevent the dropping of the target incase of power suspension. For DC potential of the target, self-biasgenerated by Rf power supply 4113 may be used. In case the target ismade of metal material, it is also effective to close the switch 4111,to connect DC power supply 4112 and to control the potential. 4116 is acircuit having the function similar to that of 4106, and it is openedonly to the frequency f_(T) of RF power supply 4113 and is grounded tothe other frequency. However, it is to DC. The power of high frequencypower supply (frequency f_(w)) to control the wafer potential is usuallylow, and 4116 may not be necessarily furnished. Normally, it ispreferable to use the frequency lower than that of RF power supply 4103connected to wafer, e.g. 13.56 MHz. This is to obtain higher sputteringspeed by generating higher self-bias on the target compared with thewafer. This is not the case when target potential is controlled by DCpower supply 4112.

In the embodiments of the invention, therefore, RF power supply withlower frequency (f_(T)) is used on target side to increase thesputtering speed, and RF with higher frequency (f_(w)) is used on thewafer side to decrease the bias of wafer. Thus, the damage to wafersubstrate is reduced and the quality of thin film can be controlled. Thecontrol of the quality of actual thin film will be described later.Here, it is set that f_(T) =13.56 MHz and f_(w) =100 MHz, while this isonly an example, and the combination of the other frequencies may beused.

Also, the embodiment as shown in FIG. 6 may be used as the target holderof the electrostatic chuck type. RF power is inputted to the targetholder electrode 415 by capacitive coupling through thin insulating film409, while DC power supply 4109 is inputted alone to the target holderelectrode 415 through high frequency filter 4102. This makes it possibleto prevent the application of high DC voltage on the capacitor, which isused in the circuit such as 4116, and reliability is increased. The samearrangement may be taken for the wafer.

In the above, description has been given on the control of potential ortarget (or wafer) by a DC power supply. Description will be given belowin connection with FIG. 7 on the case where control is performed withoutrelying on DC power supply. When LC circuit is furnished to meet theconditions:

    2πf.sub.2 =1/(L.sub.1 C.sub.1).sup.1/2

    2πf.sub.T =1/(L.sub.2 C.sub.2).sup.1/2,

the impedance to f_(w) when seen from the wafer side is 0, and thecircuit is short-circuited to f_(w). On the other hand, the impedance tof_(T) as seen from the target side is turned to 0, and the circuit isshort-circuited to f_(T). Therefore, if f_(T) is taken to 13.56 MHz forexample, the application of 13.56 MHz on the wafer can be prevented, andthe ion energy to hit wafer can be accurately controlled. It ispreferable to provide LC circuits symmetrically as shown in FIG. 8.

In the above, the electrostatic chuck method was used as the method tosupport target, while the other methods (e.g. the method of adsorb bymagnetic force) may be adopted.

The detailed description of the sputter chamber for metal thin film(FIG. 3) has been described in the above. Although not described above,a mechanism such as shutter of target may be provided in the sputterchamber, but it is not necessary for the equipment of the presentinvention. Namely, the equipment itself is suitable for ultra-highvacuum condition, and there is no need to perform frequent cleaning forthe target surface because ultra-high purity gas is used. If necessary,it may be performed with the gate valve 105a closed. Because thecontaminated layer of the surface is a layer adsorbed with smallquantity of moisture, the power of RF power supply 4113 may bedecreased, and the surface sputtering may be performed with a bias valuelower than the sputtering threshold value of the target material 109a.In so doing, the target material will not be necessarily deposited onthe inner surface of the chamber.

Next, description will be given in detail on the arrangement of thecleaning chamber 101c. Because basic structure is the same as thechamber 101a for forming metal thin film, description will be given inconnection with FIGS. 3 and 4.

In this case, the target material 109a having a relatively largethreshold to generate the sputtering such as Al₂ O₃, SiO₂, Si₃ N₄, AlN,etc. is used. The frequency of RF power supply 4113 to be applied on thetarget may be the value higher than 13.56 MHz used for metal thin film,e.g. 100 MHz. However, it is recommended to set self-bias value to 10-20V, and a higher frequency, e.g. 200 MHz or more, may be used to generatehigh density plasma. For precise control of wafer potential, it isimportant to use high frequency entering to the target side and not todeviate wafer susceptor potential. Therefore, it is important to selectf_(T) and f_(w) so that they are not in relation of an integer multiple.If f_(T) =100 MHz, for instance, it is desirable that f_(w) =210 MHz. Inso doing, high density argon ions can be generated without sputteringthe target material 109a. Argon ions thus obtained are irradiated on thesurface of wafer 404 placed on the wafer holder 405. The irradiationenergy of the argon ions is determined by self-bias generated on waferby RF power supply of 4103. The objects of cleaning are primarily a verythin natural oxide film layer or adsorbed molecule layer--particularly,adsorbed molecule layer of moisture, and argon particles with kineticenergy of several to 30 eV may be irradiated.

Therefore, it is necessary to adjust RF power supply 4103 or thematching circuit 4104 in such manner that self-bias value generated onwafer is 30 V. To generate such relatively small self-bias value withhigh controllability, it is desirable to use high frequency, e.g. 100MHz, for RF power supply 4103. Of course, 200 MHz or more may be used,but it is important to use a value different from target frequency f_(T)in order to prevent the interference between target and wafer. This isalso to have DC potential of target and wafer, which can be controlledindependently to the optimal values.

For the target 109a, description has been given in the above exampleonly for the case where insulating material is used. However, it may bethe material having conductivity such as Si if self-bias value can beset to the value low enough not to generate sputtering. Also, becausethere is no need for frequent exchange with the target material 109a,load lock exchange mechanism for the target may not be furnished.

Argon is used, but H₂ O, He, etc. may also be used. Particularly, whenthe argon gas added with H₂ gas is used for cleaning, it is possible toremove the moisture adsorbed molecule layer by the irradiation of argongas and to effectively remove the carbon atoms adsorbed on Si surface byH ions. If the molecules of impurities such as H₂ O, O₂, etc. areintermingled in gas even if in small quantities, the ultra-high impuritygas supply system (FIG. 5; 504) must be used because the surface may becontaminated. However, if special care is not taken on the gas systemused and the molecules of impurities such as H₂ O, O₂, etc. arecontained in small quantity, it is effective to add H₂ by 1-30% to argongas. This is because the oxygen radicals generated in a plasmaatmosphere are bonded with hydrogen atoms before reacting with thespecimen surface.

The adding of hydrogen gas is not limited to the case of the cleaningchamber 101c, and it goes without saying that the same effects can beobtained in the chambers for forming thin film such as 101a, 101b, etc.

The cleaning chamber as described above is provided with the function toirradiate ions with kinetic energy of several to 30 eV on the specimensurface, and it is very important to obtain the better interface for theformation of multilayer thin film structure. Moreover, the substrate onthe basis is not damaged because low energy ions are used. Above all,there is no need to increase the temperature of substrate for thiscleaning, and it can be performed at normal temperature. Therefore theheater 413 need not be furnished.

In the above, description has been given of the case where RF 4103 isapplied on the wafer susceptor. For more accurate control of the energyof argon ions, however, it is preferable to use the circuit as shown inFIG. 7. That is, the values of L and C are selected to have the relationof 2πf_(T) =1/(LC)^(1/2) (f_(T) is the frequency of RF to be applied onthe target.) Then, the LC circuit is turned to the state of parallelresonance status, and when the earth 4203 is seen from wafer holder4201, impedance is and the potential of the wafer approaches infinitelyto the potential of plasma. Namely, the energy of argon ions irradiatedon wafer approaches 0. Accordingly, if the values of L and C are alittle deviated from the value satisfying the above conditions, theenergy of argon ions can be accurately controlled even in the range of0-5 eV.

The effect of the adoption of this cleaning chamber will be describedlater in connection with experimental data.

Next, the oxidation chamber 101d will be described in detail. Becausethe oxidation chamber has the same basic arrangement as the chamber formetal thin film 101a, description will be given here in connection theFIGS. 3-5. Ultra-high purity argon and oxygen gas can be introduced intothis chamber from the gas supply system 504. In case Al is oxidized forexample, the wafer is heated by the heater 413, and the temperature ofthe wafer can be set to any value within the range of 100°-450° C.

By heating the silicon wafer 404 having Al film on its surface at 400°C. for about one hour in a high purity oxygen gas atmosphere, Al₂ O₃thin film of about 30Å can be formed through direct thermal oxidation ofaluminum. Film thickness is not increased and remains at constant valueeven when oxidation time is extended. Direct oxidation of aluminum maybe performed by introducing oxygen gas into chamber and by increasingwafer temperature under atmospheric condition. Or, temperature may beincreased at first under vacuum condition and oxygen gas may beintroduced thereafter. Oxygen pressure may be introduced at a reducedpressure atmosphere lower than atmospheric pressure, or at a higherpressure. To perform reduced pressure oxidation, exhaust is performed bya vacuum exhaust system while passing oxygen gas, and oxygen pressuremay be adjusted, or it may be diluted with argon gas. Further, it ispreferable to perform annealing for about 30 minutes at 400° C. in anargon atmosphere or in a vacuum atmosphere before oxidizing by oxygen.This is because argon gas of several ppm is contained in Al thin filmproduced by sputtering and this is released from the film and alsobecause oxide film of better quality can be obtained by thermaloxidation of Al after degasification. It is not always necessary tofurnish the target (FIG. 3; 109a), provided in the oxidation chamber,target holder 4041 and high frequency power supply 4113 as well as highfrequency power supplies (4103, 4104) connected with wafer holder.However, these are required in the following cases: For example, in casefilm thickness of Al₂ O₃ layer must be higher, e.g. 50Å or more, Al₂ O₃target is used as 109a, high frequency power supply of 13.56 MHz is usedas 4113, and a power supply of 100-200 MHz is used as 4103. When Al₂ O₃is formed by sputtering on Al₂ O₃ formed by thermal oxidation, thickfilm can be obtained. By applying bias by using high frequency powersupply 4103 of 100-200 MHz on wafer holder (FIG. 3; 107a), dense Al₂ O₃film with high characteristics can be formed. In this case, theinterface between Al₂ O₃ and Al thin film is formed by thermaloxidation, and the interface has more stable characteristics than theinterface when Al₂ O₃ film is formed by sputtering of the conventionalmethod.

Next, description will be given of the sputter chamber for insulatingthin film 101b. This chamber also has the same basic arrangement as thesputter chamber for metal thin film, and description will be given herein connection with FIGS. 3-5. Major difference between this chamber 101band the sputter chamber for metal thin film 101a is that the targetmaterial 109b is an insulating material. Therefore, in caseelectrostatic chuck holder is used on the backside of the target, it isnecessary to furnish electroconductive material such as Al, Mo, or W.Because bias cannot be controlled by DC power supplies (4108, 4112),these DC power supplies are not needed. However, it is convenient forforming metal film by sputtering if the target exchange mechanism isfurnished to exchange insulating material and metal target. In thiscase, it is possible to perform bias control using DC power supplies(4108, 4112). In any case, it is desirable to open or close the switches(4107, 4111) according to each purpose.

Finally, it should be pointed out that, in the leaf processing equipmentof this type, it is necessary to process at high speed, i.e. the timefor one processing on one wafer must be within one minute. Specifically,in case metal thin film such as Al is formed on a wafer, it is necessaryto finish the complete process within one minute, including the time toinsert it into and to take it out from the process chamber 101a. If itis supposed that the time required for film forming is 30 seconds, thetime for opening and closing of gate valve 105a, moving in and out ofwafer, and the setting of process condition is 30 seconds at the most.To cope with such requirements, a gate valve capable to open or closewithin 0.5 second is required. FIG. 1 does not give the detailedstructure of load chamber 102 and unload chamber 103, while thesechambers are naturally provided with a wafer cassette to store severaltens of wafers and also with a mechanism to receive and deliver wafersfrom and to the electrostatic chuck transport mechanism. For suchreceiving and delivering operations, high speed gate valve capable toopen or close within 0.5 second must be used as the gate valves 105e,105f, etc.

As the high speed gate valve, it is preferable to use the valve as givenin FIGS. 9-12. FIG. 9 is a sectional view of a high speed gate valveused for the equipment of this invention, and it matches with the statuswhere the gate between the process chamber 101a and the transportchamber 104 is closed. 1401 is a Ti thin plate with thickness of 0.2-0.5mm. To open or close this, a mechanism as shown in FIG. 10 may be used.FIG. 10 is a drawing of the gate valve of FIG. 9 as it is seen frombelow, and Ti thin plate 1401 is supported by two arms 1402 and 1402' attwo points 1403 and 1403'. 1404 and 1404' are the pins to attach thearms on the chamber, and the arms are moved with these pins assupporting points. In other words, by moving the arm 1402, 1401 ismoved. Here, it is important that 1401 is a thin plate (e.g. Ti thinplate), and it has a very low weight. Accordingly, it can be moved athigh speed by a simple mechanism as shown in FIG. 10. To reduce theweight of 1401, the thickness of Ti may be decreased to less than 0.1 mmand it may be reinforced by attaching it on a plastic plate. Or thesurface of reinforced plastics may be coated with a metal material suchas Ti, Mo, W, etc. to obtain a lightweight and durable plate. Even whensuch a thin plate is used, the pressure in both chambers separated bythin plate 1401 is several Torr at the most, and there arises no problemwith strength.

The material for the thin plate is not limited to Ti, and duralmin orother materials may be used. Also, it is preferable that the surface ofthin plate 1401 or the sealing portion 1405 has R_(max) of less than 0.1μm.

In FIG. 9, 1405 represents a vacuum sealing portion, and its enlargedview is given in FIG. 11. In this figure, 1406 is made of insulatingmaterial and is fixed on the wall of the chamber 1407. 1408 is a metalelectrode and is connected with one of the electrodes of the DC powersupply (not shown). The other electrode of this DC power supply isconnected with the gate valve 1401. 1409 is an insulating materialhaving thickness of 10 to several hundreds of μm. When voltage ofseveral hundred volt is applied between electrodes 1408 and 1401, gatevalve is pulled by electrostatic force, and vacuum sealing is achievedby this force. Accordingly, a material with elasticity should be used as1409. Mechanical strength and suction force are actualized by thearrangement as described above. An airtight sealing material 1411 isprovided. However, it may not be necessarily provided because sufficientairtightness can be furnished even when the sealing material is notprovided. It is preferable not to provide it from the viewpoint toprevent gas release from the sealing material.

The high speed gate valve of FIGS. 9-12 is distinguished from theconventional example by the new features that high speed opening andclosing operations can be achieved by lightweight gate valve 1401 andthat the electrostatic chuck principle is adopted for vacuum sealing andmechanical force is not used. This has made it possible to extensivelyshorten the time required for opening and closing to 0.2 to 0.5 secondscompared with several to several tens of seconds in the case of aconventional type gate valve.

The gate valve as used here can be used only in case the chambers atboth ends of the gate valve are under a vacuum condition of several Torror less. It is not usable because of its strength in case one of thechambers returns to atmospheric pressure. In such case, lightweight gatevalve 1401 may be opened as shown in FIG. 12, and the gate valve 1410 toseal by conventional mechanical force may be closed. The gate valves of106a-106d in FIG. 1 are used only in case the chambers at both ends areunder a high vacuum condition. The gate valve such as 1410 is used onlywhen the chamber is returned to atmospheric pressure for maintenance,for example, and high speed gate valve can be used at all times duringthe process. Also, for the gate valves 105e and 105f, high speed gatevalve may be used at all times for the insertion and the removal ofwafer between load chamber or unload chamber (102, 103) and thetransport chamber 104. However, when the wafer is placed into or removedfrom the equipment, it is necessary to return 102 and 103 to atmosphericpressure, and conventional type gate valve 1410 may be used althoughopening and closing speed is slow. This operation is necessary only whenthe wafer is mounted or removed from the equipment in batch, and thisdoes not result in longer wafer processing time.

Also, description has been given on the case where 4 chambers arecombined together, while such combination may be changed when necessaryor the chambers may be increased or decreased.

OTHER EMBODIMENTS

In the embodiments as described above, the wafers are placed into andremoved from each reduced pressure chamber by moving the wafer susceptor(to the vertical direction in the case of FIG. 1). Here, descriptionwill be given in the case where the wafer susceptor remains in astationary condition.

In this embodiment, a moving arm having gripping means to grip the waferat its tip is provided at the position face-to-face to the gate valves105a to 105c of each of the reduced pressure chambers 101a to 101c. Themoving arm traverses the transport chamber 104 and moves back and forthtoward the reduced pressure chambers 101a to 101c. This moving arm canreceive and deliver the wafer by the gripping means at the tip.

Next, description will be given on an example of the procedure totransport wafers in this embodiment.

First, the wafer 106e to be processed is placed on the wafer holder 107ein the load chamber 102. This wafer 106e is held by the gripping meansat the tip of the moving arm 130e, and the moving arm is moved forwardinto the transport chamber 104. Gate valve 105e is opened, and themoving arm 130e is moved forward. The wafer 106e is delivered to thetransport vehicle 512 waiting in the transport chamber 104. Afterdelivery, the moving arm 130e moves back and the gate valve 105e isclosed. On the other hand, upon receipt of the wafer 106e, the transportvehicle 512 moves to the front of cleaning chamber 101c on the track511. It stops in front of the cleaning chamber 101b, and gate valve 105cis opened. Moving arm 130c moves forward and grips the wafer on thetransport vehicle. With the wafer gripped, the moving arm 130e movesfurther forward and delivers the wafer to the wafer holder 107c in thecleaning chamber 101c. After delivery, the moving arm moves back, andthe gate valve 105c is closed. The transport of wafers between thereduced pressure chambers may be performed in the same way. Thus, thewafer can be transported without breaking the vacuum condition.

Although not shown in the drawings, an exhaust system is connected tothe transport chamber 104, reduced pressure chambers 101a to 101c, loadchamber 102, and unload chamber 103.

By this invention, it is possible to obtain various types of thin filmlaminated structures with high reliability and characteristics to beused for ultra-high density integrated circuits. As the result,integrated circuits with higher degree of integration and with highreliability can be produced.

What we claim is:
 1. A gate valve for a thin film forming apparatus,said apparatus including two adjoining low pressure chambers and a wallseparating said chambers, said wall including an aperture, said gatevalve comprising:a thin plate, said thin plate having a plate surface;drive means operatively associated with said plate for moving said platein a direction substantially parallel to said plate surface whereby saidthin plate can selectively cover and uncover said aperture; and meansfor applying a direct current voltage between said thin plate and saidwall.
 2. The gate valve of claim 1, wherein said plate surface ispolished and is disposed facing a part of said wall which surrounds saidaperture.